A 2-alarm fire that gutted portions of an apartment building in southwest Houston Wednesday morning was intentionally set, officials said.

The blaze broke out about 5 a.m., at 10100 Sapling Way near Gustine, according to the Houston Fire Department. The fire was extinguished about half of an hour later.

No injuries were reported.

A passerby spotted the flames. When firefighters arrived they saw heavy fire coming from the building.

The fire charred vacant apartments but families with small children were in adjacent units when the flames erupted. A Good Samaritan who lived at the building alerted other residents, who all got out safely.

Investigators said they later determined the fire was intentionally set. Someone used a flammable liquid to torch the exterior of the vacant apartments.

A dog trained to find accelerants alerted at several spots outside the building, including a storage closet on the second floor balcony.

Anyone with information about the fire is urged to contact Houston Fire Department arson investigators at 713-247-8900 or Crime Stoppers at 713-222-TIPS